1. Whole-cell patch-clamp recordings were used to investigate possible age-related changes in K+ currents of type 1 carotid body cells isolated from the rat. K+ current density increased with age, as measured in cells isolated from 4-day-old, 10-day-old and adult rats (> or = 5 weeks old). 2. The proportion of current reversibly inhibited by high [Mg2+] (6 mM), low [Ca2+] (0.1 mM) solutions, indicative of the proportion of current attributable to activation of Ca(2+) -sensitive K+ (KCa) channels, was significantly smaller in cells of 4-day-old rats compared with 10-day-old rats, despite inward Ca2+ current densities being similar in these two age groups. Inhibition of K+ currents by high [Mg2+], low [Ca2+] solutions was similar in 10-day-old and adult type 1 cells. 3. Hypoxia (PO2, 16-23 mmHg) caused reversible reductions in type I cells from rats of all age groups. However, reductions seen in cells of 4-day-old rats were significantly smaller than those seen in cells of 10-day-olds and adults. The degree of hypoxic inhibition in these latter two groups was not significantly different. 4. In the presence of high [Mg2+], low [Ca2+] solutions, hypoxia (PO2, 16-23 mmHg) was without significant effect on residual K+ currents in cells from all age groups. 5. These observations indicate that K+ current density increases with postnatal age in the rat. Between days 4 and 10, there appears to be a predominant enhancement of KCa channels, and over the same age range hypoxic sensitivity of K+ currents increases. Our findings demonstrate that this latter observation arises because hypoxia selectively inhibits KCa channels in cells at all ages studied. These results suggest an important role for KCa channels in postnatal maturation of hypoxic chemoreception in the rat carotid body.